Neptune
is the last of the four gas giants. It was the first planet to be discovered
by mathematics, and it has the fastest known winds in the solar system.
Even though it is one and a half times further away from the Sun
than Uranus, it is practically the same temperature.
And even though it is not the most distant object in the solar system,
one of its moons is the coldest object int he whole solar system.

Neptune is invisible to the
naked eye. It is named after the Roman god of the sea, because of its
blue-green colour. Neptune has a day which is 16.3 hours long, and it
takes 163 years for a complete orbit.

Neptune was the first planet
to be discovered by mathematics. After Uranus had been accidentally discovered,
its past and future paths were worked. When the mathematically calculated
past path was compared to past photos of Uranus, it was found that there
Uranus had not been where it was supposed to have been. And as the years
went by, Uranus began to move away from its calculated future path! Obviously,
there was another planet, a mysterious 8th planet, pulling it away from
its calculated path.

German and English mathematicians
came up with very similar predictions for the orbit, mass and position
of this 8th planet. And on the 23rd of September, 1846, Neptune was found
within 1o of its predicted position. Neptune is about one and a half times
further out from the Sun than Uranus. But Uranus is the only one of the
four gas giants to lack a large source if internal heat. Neptune emits
2.8 times more heat than it gets from the Sun. So Neptune has an internal
powerhouse to drive its weather. And unlike Uranus, which doesn't have
any real weather, Neptune has clouds and storms. In fact, Neptune is the
windiest place in the whole solar system, with winds up to 2,100 km/hr.
It has white clouds of methane,which scurry along some 50 km above the
cloud tops. Neptune is very similar to Uranus in may ways. They are practically
the same size - they are closer in volume than the other pair of sister
planets, Earth and Venus.
They are the same shade of blue, because they both have methane in their
atmospheres. They both have a very even surface temperature - less than
a 2oC variation from one pole to the other. They are at the same temperature
(-214oC). (Even though Uranus is closer to the Sun, it does not have a
hot interior, so the surface temperatures are the same). They each have
a solid core just a little larger than the Earth tucked away at their
centres.

But Neptune has weather. It
has a Great Dark Spot, about 14,000 km by 6,500 km (big enough to swallow
half the Earth). And when Voyager 2 zipped by on June 1989, a few more
differences emerged. Neptune has only 4 rings, while Uranus has 9. And
Neptune has only 8 moons,while Uranus has 15. But one of those Neptunian
moons is very strange.

Triton is one of only two moons
in the solar system to go around its mother planet in the opposite direction
of its rotation. Triton is also the coldest body in the known solar system
- only 38oC above absolute zero, or -235oC. It also
has strange plumes that rise vertically some 8 km above the surface, and
then get blown horizontally some 150 km. The big question about these
plumes is are they volcanic (erupting from the surface), or are they dust
devils (lifted by the wind)?. They would have to be very special dust
devils, because the atmospheric pressure on Triton is some 15 millionths
weaker than Earth's, or 400 times weaker than Mars'.